Rule of thumb; anything endangered or protected is not legal. Any migritory birds are out.

Here is the CA state page, it's kind of general. I recall a very detailed booklet but it's been years since I read it. If you are bidding on eBay or something like that, they generally have a disclaimer saying it's your responsibility to find out if it's legal.

www.dfg.ca.gov/enforcement/taxidrmy.html

Retailers are supposed to follow the same rules, but some just don't realize that they are also subject to the same laws. You can always quiz the shop and ask if it's legal.